Recent progress in devices utilizing radio waves, such as mobile phones, wireless LAN (Local Area Network), GPS (Global Positioning System), and automotive radar has been prolific. With the significant increase in the volume of information transmitted, telecommunication signals are requiring higher frequencies, and telecommunication devices are being downsized. Thus, there is now a strong demand for RF devices having smaller and more lightweight electronic components which transmit information at high speeds.
State-of-the-art telecommunication devices use high-speed microprocessors working at operating frequencies exceeding 500 MHz, typically from 1-10 GHz, and corresponding signal frequencies have been increasing rapidly in order to process a larger volume of information in a shorter time. One of the problems with devices handling high-speed pulse signals is delay on the printed circuit board. In general, printed circuit boards comprise an electrically insulating dielectric substrate upon which is disposed a layer of a metal. The metal can be laminated, glued, sputtered, or plated on to the printed circuit boards' substrate. As signal delay time increases in proportion to the square root of the dielectric constant of the circuit board, the choice of this dielectric substrate can have a significant impact on processing and transmission speed.
Printed-circuit boards also dissipate electromagnetic energy during signal transmission, and this effect is magnified at the higher frequencies required in modern wireless and broadband applications. This transmission results in energy loss from the electronic component in the form of heat, which can further compromise the speed and efficiency of the device.
Two properties that play a significant role in high-speed circuit design are the dielectric constant (Dk) and dissipation factor (Df) of the dielectric substrate (i.e., composite resin). Transmission loss can be reduced when one or both of Dk and Df of the composite resin are lowered. Therefore, in many applications, it is critical to choose a dielectric substrate having both a low dielectric constant and low dissipation factor in order to provide the signal transmission speeds required for state-of-the-art information processing.
Lowering the dielectric dissipation factor can have a substantial effect on the performance of printed circuit boards in high speed applications. Thus, manufacturers of, e.g., high quality internet servers and RF boards require dielectric substrates made of composite resins with a Df (i.e., loss tangent) of 0.006 or less, preferably 0.005 or less, at 10 GHz. This property allows for the fabrication of printed circuit boards that can be used for, e.g., high-frequency radio antennas and high end server applications. Additionally, manufacturers of broadband devices desire a substrate with a loss tangent that is unchanged over a wide frequency range. Accordingly, it is desirable to provide a composite resin that not only has a low loss tangent at 10 GHz, but that also has a similarly low and relatively constant loss tangent at lower frequencies, e.g., 5 GHz and 1 GHz.
Moreover, the standards imposed on the dielectric substrate employed in printed circuit boards are stringent. Ideally, the composite materials provide both excellent electrical and excellent mechanical properties. Suitable materials must be strong, easy to handle, and have a minimum of defects on the scale of several microns or less. It is also required that they have robust thermal stability, good processability, high peel strength, good moisture resistance, and UL-94 V0 flame resistance.
Various types of materials are known to exhibit low dielectric constants, including thermoplastic resins such as polyolefins, vinyl chloride resins, fluorine resins, syndiotactic polystyrene and aromatic polyester resins, as well as unsaturated polyester resins, polyimide resins, epoxy resins, bismaleimide-triazine resins (BT resins), polyphenylene ether resins (PPE) and crosslinking PPE resins, polyvinylbenzyl ether resins, benzocyclobutene resins and the like. However, when employed alone, these resins are not suited for use in electronic components intended for operation with high-frequency electromagnetic waves because it is difficult to achieve a dielectric dissipation factor of 0.006 or less (at 10 GHz), while satisfying all of the electrical and mechanical properties required for electronic components, such as heat resistance, chemical resistance, insulation, low dielectric loss tangent and low moisture absorption.
Thus, it is an object of the present invention to provide thermosetting resin compositions having a low dielectric constant (Dk) and a low loss tangent (Df), for use in the manufacture of, e.g., prepregs and printed circuit boards, and in the manufacture of radomes (a dielectric housing for antennas used in aerospace applications). It is a further object of the present invention to provide thermosetting resin compositions suitable for components intended for use in high-speed state-of-the-art electronic devices, and which possess robust thermal stability, good processability, high peel strength, good moisture resistance, a high Tg and UL-94 V0 flame resistance.